Commercial satellites have historically been optimized for broadcast applications, where data are transmitted from a broadcast center on the earth up to a satellite in space, and the satellite retransmits these signals down to a population of receive-only earth stations or satellite terminals on the earth. Traditional broadcast satellites are characterized by two features. First, traditional broadcast satellites provide “one-way” communications, such that the recipient of the data (i.e. the end-user) is equipped with a receive-only terminal that has no ability to transmit a signal back up to the satellite. Second, traditional broadcast satellites are designed for wide geographic coverage using antennas or combinations of antennas on the satellite with beams that cover large regional, national, or continental areas.
A typical business goal for traditional broadcast satellite operators is to provide as much data as possible (e.g., hundreds of television channels) to a large number of end-users or customers. For content of national or international interest (e.g., televised sports, movies and news), a satellite operator may choose to broadcast the same data to an entire country or even to an entire continent. A video broadcast satellite, with a single antenna beam covering the continental U.S. and providing hundreds of television channels to U.S. customers, is a good example of a traditional broadcast satellite. For regional content, some broadcast satellites have several antenna beams that effectively divide the earth terminal population into large regional groups such that certain combinations of the broadcast data content are transmitted to each group. In both cases, the broadcast satellite system provides one-way communications to customers over a large geographic area.
Using a traditional broadcast satellite with antenna beams covering entire national or large regional areas to private communications with a single terminal somewhere in the coverage area is not an efficient approach for network-access satellite services. For example, if a customer with a two-way earth terminal located in New York wants to establish a private two-way connection to the Internet, transmitting energy from a satellite over the entire continental U.S. to send information to a single customer in New York would be an inefficient use of limited and costly satellite resources.
In recent years, satellite operators have used satellites to provide network-access services (e.g., telephony, private networks, and Internet access) to a large population of end-users or customers. In modern network-access satellite communications systems, end-users are equipped with earth terminals that both receive signals from a satellite and also transmit signals back up to a satellite. Modern network-access satellite systems are architecturally different from traditional one-way broadcast satellite systems in that each earth terminal is, in effect, carrying on a two-way private conversation with the satellite network and generally has no interest in “hearing” signals being transmitted to and from any other earth terminals on the network.
A satellite with a more highly focused antenna beam limited in area to an individual customer's immediate local area s a much more efficient way for transmitting data to this particular customer than a traditional broadcast satellite. Similarly, in the earth-to-space direction, if a receiver on a satellite is focused in on a much narrower geographical region that covers just the customer's immediate area, less power is required for that customer's earth terminal to transmit information to the highly focused receiver on the satellite.
Modern network-access satellites are characterized by two features. First, modern network-access satellites provide “two-way” communications between satellites in space and terminals on the earth that have both transmit and receive capability. Second, modern network-access satellites are designed with antennas that cover the geographic area of interest on the earth with many smaller antenna beams, often tightly packed together to provide fall coverage across the area of interest without any gaps. For example, some modern network-access satellites transmit tightly packed clusters of small antenna beams that collectively cover a large geographic area, such as the continental U.S. For two-way network-access communications, by using a number of “spot-beams” over their coverage area, spot-beam satellites have significant advantages over satellites that have a single beam over the coverage area. For example, spot-beam satellites require less satellite transmitter power per customer. As another example, less transmitter power is required for earth terminals to transmit to spot-beam satellites, allowing for smaller and less expensive earth terminals. Additional advantages include the ability to reuse the same frequency bands and channels throughout the spot-beam pattern and associated coverage area, dramatically higher non-broadcast capacity per satellite to provide more compelling services to more customers, and dramatically lower satellite cost per customer. For example, the capacity of a spot-beam satellite to support a large population of end-users may be greatly enhanced by frequency reuse techniques, whereby the same frequency bands and channels are used over and over again in non-adjacent spot-beams. For example, a satellite operator may have a 500 MHz bandwidth allocation for space to earth transmissions assigned by the appropriate regulatory authority. In a single beam network architecture, this satellite operator is limited to 500 MHz of total transmission bandwidth. The transmission bandwidth may be increased by dividing this bandwidth into multiple channels, such as for example, four 125 MHz channels, and assigning one channel to each of numerous spot-beams. In this example, if the satellite utilizes 100 spot-beams, this satellite operator could utilize 12,500 MHz of total transmission bandwidth. This ability to apply frequency reuse techniques to greatly increase the capacity of a satellite network is a technical advantage of the spot-beam satellite architecture.